Petroleum products generally have a viscosity which exhibits large variation with change in temperature, namely have temperature dependence of viscosity. For example, lubricating oils used for automobiles or the like preferably have low temperature dependence of viscosity. In order to decrease the temperature dependence of viscosity, a certain kind of polymers soluble in lubricating oil bases is used in the lubricating oils as a viscosity index improver. In recent years, ethylene•α-olefin copolymers have been widely used as viscosity index improvers and, further, the ethylene•α-olefin copolymers have been variously improved in order to further improve the properties of the lubricating oils (refer to Publication of WO00/34420).
The viscosity index improver is generally used so that the lubricating oil retains a proper viscosity at high temperatures. Recently, the quality standard of lubricating oils typified by engine oils has been advanced, and therefore, a polymer for viscosity index improvers capable of reducing the viscosity elevation at low temperatures (having excellent low-temperature properties) has been particularly desired. In the lubricating oil application, it is desirable to reduce the polymer concentration as low as possible in order that the lubricating oils have more excellent low-temperature properties. Use of a polymer having a higher molecular weight is known for the above reason and also for economical advantage. However, if the molecular weight is increased and the addition amount of the polymer is decreased, a problem appears in that the shear stability is decreased.
Furthermore, common lubricating oils contain a paraffin type mineral oil, and the paraffin type mineral oil contains a paraffin wax component in an amount of 1 to 5%. The paraffin wax forms a plate-like crystal at low temperatures and further absorbs oil components to impart a three-dimensional network structure so that the fluidity of the whole lubricating oil is greatly lowered. A pour-point depressant is simultaneously used in order to improve the fluidity by making this plate-like crystal into an amorphous state. However, the effects of the pour point depressant vary largely depending on the kind of lubricating oil base and therefore it is required to select a pour point depressant suitable for each lubricating oil base.
In application in engine oils for automobiles and industries, gear oils (including ATF), hydraulic fluids and the like, the utilization of highly purified lubricating oil bases, such as oils classified in groups (II) and (III) has increased in place of oils classified in the group (I) which have been conventionally widely used as a lubricating oil base, in order to cope with the sophistication of required properties associated with setting new standards and the enforcement of environmental regulations.
The engine oil application has a large problem in that low-temperature viscosity as measured by Mini-Rotary Viscometer (parameters of oil pumping properties), which is a major standard, is increased.
The present inventors found that high molecular weight ethylene•α-olefin copolymers are suitable as viscosity index improvers for lubricating oils having excellent low temperature properties and economical properties, but if the molecular weight thereof is too high, the solubility in the lubricating oil base is lowered and the low temperature properties become worse. They also found that the use of highly purified lubricating base oil produces a tendency that the solubility of the high molecular weight ethylene•α-olefin copolymers is further lowered.
Under the circumstances, the present inventors have been diligently studied the above problems and found that they are solved by using a viscosity index improver which is an ethylene•propylene copolymer, having an ethylene content, molecular weight, molecular weight distribution and melting point in specific ranges, and optionally using a pour point depressant as needed. Thus, the present invention has been accomplished.
It is an object of the present invention to provide a lubricating oil composition having excellent viscosity properties at low temperature and thickening power suitable for engine oils for automobiles and industrial applications, gear oils, shock absorber oils, hydraulic fluids. The invention further provides a lubricating oil for internal-combustion engines formed from the composition.